Dichotic listening performance and effort as a function of spectral resolution and interaural symmetry

The Relationship Between Spatial Release From Masking and Listening Effort Among Cochlear Implant Users With Single-Sided Deafness

OBJECTIVES

To examine speech intelligibility and listening effort in a group of patients with single-sided deafness (SSD) who received a cochlear implant (CI). There is limited knowledge on how effectively SSD-CI users can integrate electric and acoustic inputs to obtain spatial hearing benefits that are important for navigating everyday noisy environments. The present study examined speech intelligibility in quiet and noise simultaneously with measuring listening effort using pupillometry in individuals with SSD before, and 1 year after, CI activation. The study was designed to examine whether spatial separation between target and interfering speech leads to improved speech understanding (spatial release from masking [SRM]), and is associated with a decreased effort (spatial release from listening effort [SRE]) measured with pupil dilation (PPD).

DESIGN

Eight listeners with adult-onset SSD participated in two visits: (1) pre-CI and (2) post-CI (1 year after activation). Target speech consisted of Electrical and Electronics Engineers sentences and masker speech consisted of AzBio sentences. Outcomes were measured in three target-masker configurations with the target fixed at 0° azimuth: (1) quiet, (2) co-located target/maskers, and (3) spatially separated (±90° azimuth) target/maskers. Listening effort was quantified as change in peak proportional PPD on the task relative to baseline dilation. Participants were tested in three listening modes: acoustic-only, CI-only, and SSD-CI (both ears). At visit 1, the acoustic-only mode was tested in all three target-masker configurations. At visit 2, the acoustic-only and CI-only modes were tested in quiet, and the SSD-CI listening mode was tested in all three target-masker configurations.

RESULTS

Speech intelligibility scores in quiet were at the ceiling for the acoustic-only mode at both visits, and in the SSD-CI listening mode at visit 2. In quiet, at visit 2, speech intelligibility scores were significantly worse in the CI-only listening modes than in all other listening modes. Comparing SSD-CI listening at visit 2 with pre-CI acoustic-only listening at visit 1, speech intelligibility scores for co-located and spatially separated configurations showed a trend toward improvement (higher scores) that was not significant. However, speech intelligibility was significantly higher in the separated compared with the co-located configuration in acoustic-only and SSD-CI listening modes, indicating SRM. PPD evoked by speech presented in quiet was significantly higher with CI-only listening at visit 2 compared with acoustic-only listening at visit 1. However, there were no significant differences between co-located and spatially separated configurations on PPD, likely due to the variability among this small group of participants. There was a negative correlation between SRM and SRE, indicating that improved speech intelligibility with spatial separation of target and masker is associated with a greater decrease in listening effort on those conditions.

CONCLUSIONS

The small group of patients with SSD-CI in the present study demonstrated improved speech intelligibility from spatial separation of target and masking speech, but PPD measures did not reveal the effects of spatial separation on listening effort. However, there was an association between the improvement in speech intelligibility (SRM) and the reduction in listening effort (SRE) from spatial separation of target and masking speech.

Increased listening effort and decreased speech discrimination at high presentation sound levels in acoustic hearing listeners and cochlear implant users

The sounds we experience in our everyday communication can vary greatly in terms of level and background noise depending on the environment. Paradoxically, increasing the sound intensity may lead to worsened speech understanding, especially in noise. This is known as the "Rollover" phenomenon. There have been limited studies on rollover and how it is experienced differentially across aging groups, for those with and without hearing loss, as well as cochlear implant (CI) users. There is also mounting evidence that listening effort plays an important role in challenging listening conditions and can be directly quantified with objective measures such as pupil dilation. We found that listening effort was modulated by sound level and that rollover occurred primarily in the presence of background noise. The effect on listening effort was exacerbated by age and hearing loss in acoustic listeners, with greatest effect in older listeners with hearing loss, while there was no effect in CI users. The age- and hearing-dependent effects of rollover highlight the potential negative impact of amplification to high sound levels and therefore has implications for effective treatment of age-related hearing loss.

Reduced digit spans and ear dominance using dichotic digits in bimodal cochlear-implant users

Bimodal stimulation, a cochlear implant (CI) in one ear and a hearing aid (HA) in the other, provides highly asymmetrical inputs. To understand how asymmetry affects perception and memory, forward and backward digit spans were measured in nine bimodal listeners. Spans were unchanged from monotic to diotic presentation; there was an average two-digit decrease for dichotic presentation with some extreme cases of decreases to zero spans. Interaurally asymmetrical decreases were not predicted based on the device or better-functioning ear. Therefore, bimodal listeners can demonstrate a strong ear dominance, diminishing memory recall dichotically even when perception was intact monaurally.

How to vocode: Using channel vocoders for cochlear-implant research

The channel vocoder has become a useful tool to understand the impact of specific forms of auditory degradation-particularly the spectral and temporal degradation that reflect cochlear-implant processing. Vocoders have many parameters that allow researchers to answer questions about cochlear-implant processing in ways that overcome some logistical complications of controlling for factors in individual cochlear implant users. However, there is such a large variety in the implementation of vocoders that the term "vocoder" is not specific enough to describe the signal processing used in these experiments. Misunderstanding vocoder parameters can result in experimental confounds or unexpected stimulus distortions. This paper highlights the signal processing parameters that should be specified when describing vocoder construction. The paper also provides guidance on how to determine vocoder parameters within perception experiments, given the experimenter's goals and research questions, to avoid common signal processing mistakes. Throughout, we will assume that experimenters are interested in vocoders with the specific goal of better understanding cochlear implants.

Cochlear-Implant Simulated Signal Degradation Exacerbates Listening Effort in Older Listeners

OBJECTIVES

Individuals with cochlear implants (CIs) often report that listening requires high levels of effort. Listening effort can increase with decreasing spectral resolution, which occurs when listening with a CI, and can also increase with age. What is not clear is whether these factors interact; older CI listeners potentially experience even higher listening effort with greater signal degradation than younger CI listeners. This study used pupillometry as a physiological index of listening effort to examine whether age, spectral resolution, and their interaction affect listening effort in a simulation of CI listening.

DESIGN

Fifteen younger normal-hearing listeners (ages 18 to 31 years) and 15 older normal-hearing listeners (ages 65 to 75 years) participated in this experiment; they had normal hearing thresholds from 0.25 to 4 kHz. Participants repeated sentences presented in quiet that were either unprocessed or vocoded, simulating CI listening. Stimuli frequency spectra were limited to below 4 kHz (to control for effects of age-related high-frequency hearing loss), and spectral resolution was decreased by decreasing the number of vocoder channels, with 32-, 16-, and 8-channel conditions. Behavioral speech recognition scores and pupil dilation were recorded during this task. In addition, cognitive measures of working memory and processing speed were obtained to examine if individual differences in these measures predicted changes in pupil dilation.

RESULTS

For trials where the sentence was recalled correctly, there was a significant interaction between age and spectral resolution, with significantly greater pupil dilation in the older normal-hearing listeners for the 8- and 32-channel vocoded conditions. Cognitive measures did not predict pupil dilation.

CONCLUSIONS

There was a significant interaction between age and spectral resolution, such that older listeners appear to exert relatively higher listening effort than younger listeners when the signal is highly degraded, with the largest effects observed in the eight-channel condition. The clinical implication is that older listeners may be at higher risk for increased listening effort with a CI.

Review of Binaural Processing With Asymmetrical Hearing Outcomes in Patients With Bilateral Cochlear Implants

Bilateral cochlear implants (BiCIs) result in several benefits, including improvements in speech understanding in noise and sound source localization. However, the benefit bilateral implants provide among recipients varies considerably across individuals. Here we consider one of the reasons for this variability: difference in hearing function between the two ears, that is, interaural asymmetry. Thus far, investigations of interaural asymmetry have been highly specialized within various research areas. The goal of this review is to integrate these studies in one place, motivating future research in the area of interaural asymmetry. We first consider bottom-up processing, where binaural cues are represented using excitation-inhibition of signals from the left ear and right ear, varying with the location of the sound in space, and represented by the lateral superior olive in the auditory brainstem. We then consider top-down processing via predictive coding, which assumes that perception stems from expectations based on context and prior sensory experience, represented by cascading series of cortical circuits. An internal, perceptual model is maintained and updated in light of incoming sensory input. Together, we hope that this amalgamation of physiological, behavioral, and modeling studies will help bridge gaps in the field of binaural hearing and promote a clearer understanding of the implications of interaural asymmetry for future research on optimal patient interventions.

Time Scales and Moments of Listening Effort Revealed in Pupillometry

This article offers a collection of observations that highlight the value of time course data in pupillometry and points out ways in which these observations create deeper understanding of listening effort. The main message is that listening effort should be considered on a moment-to-moment basis rather than as a singular amount. A review of various studies and the reanalysis of data reveal distinct signatures of effort before a stimulus, during a stimulus, in the moments after a stimulus, and changes over whole experimental testing sessions. Collectively these observations motivate questions that extend beyond the "amount" of effort, toward understanding how long the effort lasts, and how precisely someone can allocate effort at specific points in time or reduce effort at other times. Apparent disagreements between studies are reconsidered as informative lessons about stimulus selection and the nature of pupil dilation as a reflection of decision making rather than the difficulty of sensory encoding.

Reconsidering commonly used stimuli in speech perception experiments

This paper examines some commonly used stimuli in speech perception experiments and raises questions about their use, or about the interpretations of previous results. The takeaway messages are: 1) the Hillenbrand vowels represent a particular dialect rather than a gold standard, and English vowels contain spectral dynamics that have been largely underappreciated, 2) the /ɑ/ context is very common but not clearly superior as a context for testing consonant perception, 3) /ɑ/ is particularly problematic when testing voice-onset-time perception because it introduces strong confounds in the formant transitions, 4) /dɑ/ is grossly overrepresented in neurophysiological studies and yet is insufficient as a generalized proxy for "speech perception," and 5) digit tests and matrix sentences including the coordinate response measure are systematically insensitive to important patterns in speech perception. Each of these stimulus sets and concepts is described with careful attention to their unique value and also cases where they might be misunderstood or over-interpreted.

Effects of better-ear glimpsing, binaural unmasking, and spectral resolution on spatial release from masking in cochlear-implant users

Bilateral cochlear-implant (BICI) listeners obtain less spatial release from masking (SRM; speech-recognition improvement for spatially separated vs co-located conditions) than normal-hearing (NH) listeners, especially for symmetrically placed maskers that produce similar long-term target-to-masker ratios at the two ears. Two experiments examined possible causes of this deficit, including limited better-ear glimpsing (using speech information from the more advantageous ear in each time-frequency unit), limited binaural unmasking (using interaural differences to improve signal-in-noise detection), or limited spectral resolution. Listeners had NH (presented with unprocessed or vocoded stimuli) or BICIs. Experiment 1 compared natural symmetric maskers, idealized monaural better-ear masker (IMBM) stimuli that automatically performed better-ear glimpsing, and hybrid stimuli that added worse-ear information, potentially restoring binaural cues. BICI and NH-vocoded SRM was comparable to NH-unprocessed SRM for idealized stimuli but was 14%-22% lower for symmetric stimuli, suggesting limited better-ear glimpsing ability. Hybrid stimuli improved SRM for NH-unprocessed listeners but degraded SRM for BICI and NH-vocoded listeners, suggesting they experienced across-ear interference instead of binaural unmasking. In experiment 2, increasing the number of vocoder channels did not change NH-vocoded SRM. BICI SRM deficits likely reflect a combination of across-ear interference, limited better-ear glimpsing, and poorer binaural unmasking that stems from cochlear-implant-processing limitations other than reduced spectral resolution.

Assessment methods for determining small changes in hearing performance over time

Although the behavioral pure-tone threshold audiogram is considered the gold standard for quantifying hearing loss, assessment of speech understanding, especially in noise, is more relevant to quality of life but is only partly related to the audiogram. Metrics of speech understanding in noise are therefore an attractive target for assessing hearing over time. However, speech-in-noise assessments have more potential sources of variability than pure-tone threshold measures, making it a challenge to obtain results reliable enough to detect small changes in performance. This review examines the benefits and limitations of speech-understanding metrics and their application to longitudinal hearing assessment, and identifies potential sources of variability, including learning effects, differences in item difficulty, and between- and within-individual variations in effort and motivation. We conclude by recommending the integration of non-speech auditory tests, which provide information about aspects of auditory health that have reduced variability and fewer central influences than speech tests, in parallel with the traditional audiogram and speech-based assessments.